1. Field of the Invention
The present invention relates generally to liquid level sensors and more particularly to a capacitive liquid level sensor especially useful for measuring the level of the liquid which undergoes changes in conductivity.
2. Description of the Related Art
Capacitive probes and sensors for measuring the level of a liquid within a reservoir are well known. Such sensors generally include a reference capacitor adapted to be fully submerged in the liquid and a measuring capacitor. As the level of the liquid elevationally varies within the gap between the plates of the measuring capacitor, the capacitance changes due to the change of the dielectric constant between the plates. By taking the ratio of the capacitance between the two capacitors, the level of the liquid may then be ascertained. The ratio of the capacitances compensates for temperature dependency by cancelling of the temperature dependency in the value of capacitance for each capacitor.
A class of such capacitive sensor has been developed to measure the level of a conductive or lossy dielectric liquid, especially where the conductivity of the liquid is initially unkown and also varies during measurement. For example, where the liquid is a lubricant used in machinery, contamination in the form of metallic particulates suspended in the lubricant significantly and undeterminably affect the conductivity of the liquid. When the sensor is inserted in a conductive liquid, conductance between the plates of the reference and measuring capacitors causes errors in measuring the capacitive ratio whereby the sensor becomes inaccurate.
Attempts have been made to correct the above problems in capacitive sensors by providing phase discrimination between the resistive and capacitive voltages. The resistive voltage arises from the inaccuracies due to the conduction between the two capacitors. Since the resistive and capacitive voltages are 90.degree. out of phase with each other, phase discrimination should be able to eliminate the resistive component. However, these prior art attempts have not met with a great degree of success. Phase discrimination permits a portion of the resistive voltage to add or subtract from the desired phase discriminated output current due to unavoidable circuit phase shifts. Since the resistive current may be generally very large, indicating a virtual short circuit between the two capacitors, the errors thus produced can be many times greater than the capacitance signal current. The results is that such a capacitive sensor system may be disadvantageously inaccurate.
In the prior art, this inaccuracy has been compensated for by the use of resistive bridge circuit wherein the parasitic conductance through the liquid forms a first resistor of a balancing bridge and a potentiometer is provided in another leg of the bridge to compensate, thereby removing the parasitic resistance from the capacitive ratio. However, the conductance of a liquid may be highly dependent upon temperature, causing the parasitic resistor to change value relative to the potentiometer setting.
Complex electromechanical systems have been proposed for the in situ adjustment of the potentiometer based upon means provided to sense the conductivity of the liquid, for example as described in U.S. Pat. No. 3,114,262. In the '262 patent, a feedback circuit from the output to the input of the bridge is discribed wherein the feedback signal is a function of the resistivity of the liquid to be measured. The feedback circuit is designed to have a negative resistance temperature coefficient. As described in column 2, lines 39-50 of the '262 patent, the feedback signal has a high effect at high temperatures and a low effect at low temperatures, such that amplifier gain is lowest at high temperatures and highest at low temperatures. The bridge operation is thus stabilized over a wide range of temperatures. A limitation of the described system is that the resistive bridge circuit and the feedback circuit must be accurately matched to the liquid being measured.
In U.S. Pat. No. 3,515,000 an attempt to overcome the above disadvantage of matching the bridge impedance and the resistivity of the liquid being measured is described. A sensor is provided wherein the measuring electrode and the common electrode are in the input circuit of an operational amplifier, the reference electrode and the common electrode are in the feedback circuit of the amplifier. The impedance in the input circuit between the measuring electrode and the common electrode will be a function of the level of the liquid in the tank in which the sensor is immersed. The feedback impedance will be a function of the characteristics of the same liquid since the reference electrode is immersed in the same liquid as the measuring electrode. The gain of the operational amplifier connected in this manner will be equal to the ratio of the reference impedance of the measuring impedance. Since both impedances are products of the electrical impedance of the same liquid, the ratio will be indepedent of the electrical impedance of the liquid and it will merely indicate the ratio of the surface areas of the measuring and reference capacitors, hence the capacitive ratio, immersed in the liquid. A limitation of the capacitive sensor described in the '000 patent is that conductance in the liquid may occur between the measuring and the reference electrodes as hereinabove discussed. If this conductance becomes large enough, the operational amplifier will be shorted out and hence always have unity gain independent of the level of the liquid to be measured.